Detection, localization and quantification of leaks related to piston machines (as may hereafter be referred to as pumps, for simplicity) are important for minimizing costs and down time related to valve and piston failures. Today's practice provides no certain way to localize leaks. A leak in one or more valves or pistons will cause a drop in the volumetric efficiency. If a pump is running at a constant speed, this reduction in volumetric efficiency also causes the mean discharge pressure to drop. However, a pressure drop can result also from leaks outside the pump or even reductions in the flow resistance not related to leaks. Such reductions in flow resistance may arise from temperature and viscosity, or they can come from a bypass of one or more flow restrictors. Therefore, a pressure drop cannot be used for localizing the leak.
Skilled operators can sometimes localize valve leaks by listening to the sound of the pump by a simple stethoscope, normally in the form of a screwdriver or a wooden stick held between the valve block and the human ear. There are however some disadvantages related to this method, such as:                The localization is uncertain, even for a trained and experienced person.        It is generally not possible to distinguish between a suction valve and a discharge valve.        The person has to make the diagnosis in hazardous area because he/she must stay very close to the pump while it is running.        The person will also be exposed to injuriously high sound pressure levels, often exceeding 100 dBA near the pump.        The diagnosis is time consuming.        The check will only be carried out at certain intervals.        
A leak in a valve or piston manifests itself by several effects that can be picked up by various sensors. The most striking changes due to a growing leak are:                The discharge pressure starts dropping, provided that the loss in pressure is not compensated by an increase in the total pump rate.        The discharge pressure from a pump starts to vary cyclically with a period equal to the pump rotation period.        The suction pressure to a pump also starts to vary cyclically with the same period.        Low frequency and cyclic vibrations increase, especially on flexible hoses, both the high-pressure hose and the low-pressure hose.        The high frequency vibration level of the pump near the leak source increases.        
Prior art include several methods for leak detection that are utilizing the first four features listed above. According to U.S. Pat. No. 5,720,598, the pressure from at least one pump in combination with the rotational speed of the pump, measured in time, are utilized to determine and analyze the pump harmonics for the presence of a defect and the type of defect. The specific pump unit having the defect is then determined.
WO document 03/087754 describes a method using a combination of active speed variation tests and harmonic analysis to both quantify and localize a leakage.
Experience has shown that the prior art methods above do not work satisfactorily in in situ environments. It is particularly difficult to pinpoint the actual valve leaking.
A leak flow in the reverse direction through a defective valve will generate high frequency vibrations in the valve block. The vibrations may be picked up by an accelerometer placed close to the leak source, for instance on the outside surface of a valve block.
U.S. Pat. No. 5,650,943 describes a method utilizing portable equipment where transducers are applied to appropriate locations in the valve system to obtain sound signals. The signals are fast Fourier transformed into valve signatures. The differential signature method is used to make determinations of valve leaks. The method is designed to evaluate whether a valve is leaking or not. It is not designed to distinguish between leaks in the suction valve and the discharge valve in a valve block. Thus, the method includes comparing obtained signals with stored signals from the actual valve, where the stored signal is obtained from a previously made database. The method will not distinguish between valves in the same valve block.
Norwegian document 20072230 discloses a method that utilizes the fact that a leak in a closed valve of a piston machine will generate high frequency vibrations in the valve block containing the leaky valve. By picking up these vibrations by accelerometers (one per valve block), and processing the vibration data together with a timing signal it is possible by use of the method to detect a leak and localize the leak source.
In situ tests have shown that the method described in Norwegian document 20072230 applies for Hex pumps where the valve blocks are widely separated so that a leak induced vibration transferred from the leaky valve block to another valve block, is small. However, field experience has also shown that said method does not work with pumps, such as quintuplex pumps, having one integrated valve block instead of split valve blocks. The reason is that the damping of the vibration within the block is relatively low. In other words, leak induced vibration can be picked up almost anywhere on the valve block and accelerometer placement will not help to localize the leak like it does in a Hex pump.
In situ measurements also indicate that the method described in Norwegian document 20072230 does not work satisfactorily on triplex pumps, although a triplex pump have separated valve blocks. A possible reason for an observed relatively strong vibration transfer between valve blocks in a triplex pump, may be the suction manifold. The suction manifold is often a relatively large pipe having flanges and it acts as an effective bridge for transfer of vibration between the three suction valve blocks.
The object of the disclosure is to overcome or reduce at least one of the drawbacks of the prior art.